Auxiliary power units on commercial aircraft are used to provide the aircraft with electrical and pneumatic power while the main propulsion engines of the aircraft are shut down during passenger embarking, baggage loading and unloading, and during maintenance and servicing of the aircraft. Auxiliary power units usually provide pneumatic power in the form of bleed air for the air conditioning and heating requirements of the interior cabin of commercial aircraft via control systems in accordance with the interior cabin's environmental requirements when the aircraft's main propulsion engines are not operating.
Operation of auxiliary power units is known to create two independent sources of engine noise; namely, noise originating from the turbine engine exhaust and noise originating from the anti-surge or excess bleed air which is present as the environmental control system routinely vents pressurized air. The anti-surge or excess bleed air is high pressure air required to prevent compressor surge in the auxiliary power unit at certain operating conditions and originates from the discharge of the auxiliary power unit turbine compressor which drives the interior cabin's environmental control system. The noise level generated by auxiliary power units during operation can be well in excess of acceptable noise levels for a given work environment. The conventional noise suppressors found in the prior art have typically been designed and intended to independently reduce either exhaust noise or anti-surge air noise at great expense and with inherent weight and volume limitations. Furthermore, the design and construction of such prior art devices have lacked the capacity or capability to be retrofitted to the auxiliary power units presently being used by existing aircraft, whether private, commercial, or military.
The present invention is directed toward overcoming the problems found in the prior art recited above.